i recommend you do this:
buy an Octopus Card at information (the card costs 150 hkd, you get 50 back if you return it). You will need an octopus card to pay for the bus, mtr, and kcr. You can deposit money on the card at the plentiful Circle K's and even use it at convenience stores or fast food joints.
to get to Kowloon: if you're on a budget, take the Airbus City Flyer. You can't miss these doubledeckers when you exit the airport for public transport. You will need your octopus card to pay the fare. The airport express is much more expensive and a taxi will be around $250hkd I think..
Where exactly are you going to be staying on Kowloon?
Lines A, E, and N all go to Kowloon but some lines don't go to certain districts.
Line A21 will take you through the major districts on Kowloon - Mongkok, Tsim Sha Tsui and Hung Hom (the last possible stop). The fare is $33hkd. This is the line I used every time I had to arrive or depart from the airport.
Let me know if you have any questions at all about HK. I lived there for 5 months this spring sem. Loved it.